E. H. L. Schwarz — Plains in (Jape Colony. 193 



the two continents in respect to some datum. The absolute 

 base level of erosion I found to lie submerged some 9,000 feet 

 in Europe and 1200 feet in South Africa. 



Beyond the absolute base level of erosion lie the plains of 

 the abyss, but we cannot legitimately include these in possible 

 land forms unless we can prove that the theory of the perma- 

 nence of ocean basins is an illusionary one. 



(7) Finally there is the plain of deposition due to subsidence. 

 Of these we have many examples along our coast, and to 

 understand them it must be remembered that our rivers run 

 out to the sea through rock gates and never form deltas. Eleva- 

 tion and subsidence are alternate motions in the earth's crust, 

 and whereas the evidences for elevation are so marked in South 

 Africa, those for subsidence are necessarily as plain. In the 

 mouths of the Knysna, Keurboom, Kowie and Buffalo rivers, 

 the rock channel has been cut to a hundred or more feet below 

 present sea-level. In the last cited case the contours of the 

 rock bottom have been accurately surveyed by Mr. J. J. God- 

 frey by means of bore-holes, and show a normal Y-shaped 

 river channel with three terraces, the deepest part being 124 

 feet below low water mark. In this lie accumulations of clay, 

 sand and shells ; the latter were sealed up while yet decompos- 

 ing and have yielded reservoirs of marsh gas which caused 

 explosions in the bore-holes when tapped. The actual river 

 bottom lies now 14 feet below low-water mark. * These sub- 

 sidence plains of deposit are naturally limited in extent, but 

 they are sufficiently large in some places to be taken into 

 account in the present connection. In the Knysna estuary 

 they form wide salt-meadows which are partially covered at 

 high water, but with very little work could be reclaimed, f 



* Schwarz, E. H. L., The Eock Channel of the Buffalo Eiver, East London, 

 Eecords Albany Museum, Grahanistown, vol. ii, p. 1, 1907. 



fThe Coastal Plateau in George, Knysna, etc., Ann. Eep. Geol. Comm., 

 p. 85, map. Capetown, 1906. 



